Gastrointestinal Tract and Respiratory System

Note the relative size and position of individual structures and organs at this early stage of development. In particular the mid-gut which is still herniated outside the ventral body wall. Below the animation is a more complete description of each system.

Gastrointestinal Tract

The gastrointestinal tract (GIT) develops primarily from the endoderm, with inputs from the mesoderm (splanchnic mesoderm) and the ectoderm (neural crest cells form its primary nerve plexus). After the endoderm is folded into a blind-ended tube in week 4, it begins to develop into the primary lining of the GIT, while the mesoderm provides its muscular wall and connective tissues components. The GIT is typically partitioned into 3 parts, based on their separate blood supplies: foregut, midgut and hindgut, with the allantois coming off the hindgut. A bud coming off the tract forms the liver; eventually buds appear that form gallbladder and pancreas. The rostral end of the tube ends at the buccopharyngeal membrane, where it interacts with ectoderm directly (no mesoderm in between). This area forms the future mouth. A similar endodermal-ectodermal interaction occurs at the tail the cloacal membrane. The section of the GIT here is called the cloaca. During week 4, the tube begins to dilate in a certain region, with the dorsal border growing more rapidly than the ventral. This establishes the greater and lesser curvatures of the stomach. The stomach also rotates 90o along the longitudinal axis. On day 22, a small endodermal thickening is visible on the ventral side of the duodenum this becomes the hepatic diverticulum that forms the liver over the next few days. The liver has an important in blood formation. As early as the fourth week, red blood cells production moves to the liver, as opposed to blood islands in the extraembryonic mesoderm.

Respiratory System

The respiratory system is endodermal in origin, initially "budding off" the foregut during week 3. This bud forms the respiratory diverticulum, at the level of the glottis between the adult oesophagus and trachea. It continues to bud in week 4, forming a pair of lung buds.